1package constant; 2 3use strict; 4use 5.006_00; 5use warnings::register; 6 7our($VERSION, %declared); 8$VERSION = '1.04'; 9 10#======================================================================= 11 12# Some names are evil choices. 13my %keywords = map +($_, 1), qw{ BEGIN INIT CHECK END DESTROY AUTOLOAD }; 14 15my %forced_into_main = map +($_, 1), 16 qw{ STDIN STDOUT STDERR ARGV ARGVOUT ENV INC SIG }; 17 18my %forbidden = (%keywords, %forced_into_main); 19 20#======================================================================= 21# import() - import symbols into user's namespace 22# 23# What we actually do is define a function in the caller's namespace 24# which returns the value. The function we create will normally 25# be inlined as a constant, thereby avoiding further sub calling 26# overhead. 27#======================================================================= 28sub import { 29 my $class = shift; 30 return unless @_; # Ignore 'use constant;' 31 my %constants = (); 32 my $multiple = ref $_[0]; 33 34 if ( $multiple ) { 35 if (ref $_[0] ne 'HASH') { 36 require Carp; 37 Carp::croak("Invalid reference type '".ref(shift)."' not 'HASH'"); 38 } 39 %constants = %{+shift}; 40 } else { 41 $constants{+shift} = undef; 42 } 43 44 foreach my $name ( keys %constants ) { 45 unless (defined $name) { 46 require Carp; 47 Carp::croak("Can't use undef as constant name"); 48 } 49 my $pkg = caller; 50 51 # Normal constant name 52 if ($name =~ /^_?[^\W_0-9]\w*\z/ and !$forbidden{$name}) { 53 # Everything is okay 54 55 # Name forced into main, but we're not in main. Fatal. 56 } elsif ($forced_into_main{$name} and $pkg ne 'main') { 57 require Carp; 58 Carp::croak("Constant name '$name' is forced into main::"); 59 60 # Starts with double underscore. Fatal. 61 } elsif ($name =~ /^__/) { 62 require Carp; 63 Carp::croak("Constant name '$name' begins with '__'"); 64 65 # Maybe the name is tolerable 66 } elsif ($name =~ /^[A-Za-z_]\w*\z/) { 67 # Then we'll warn only if you've asked for warnings 68 if (warnings::enabled()) { 69 if ($keywords{$name}) { 70 warnings::warn("Constant name '$name' is a Perl keyword"); 71 } elsif ($forced_into_main{$name}) { 72 warnings::warn("Constant name '$name' is " . 73 "forced into package main::"); 74 } else { 75 # Catch-all - what did I miss? If you get this error, 76 # please let me know what your constant's name was. 77 # Write to <rootbeer@redcat.com>. Thanks! 78 warnings::warn("Constant name '$name' has unknown problems"); 79 } 80 } 81 82 # Looks like a boolean 83 # use constant FRED == fred; 84 } elsif ($name =~ /^[01]?\z/) { 85 require Carp; 86 if (@_) { 87 Carp::croak("Constant name '$name' is invalid"); 88 } else { 89 Carp::croak("Constant name looks like boolean value"); 90 } 91 92 } else { 93 # Must have bad characters 94 require Carp; 95 Carp::croak("Constant name '$name' has invalid characters"); 96 } 97 98 { 99 no strict 'refs'; 100 my $full_name = "${pkg}::$name"; 101 $declared{$full_name}++; 102 if ($multiple) { 103 my $scalar = $constants{$name}; 104 *$full_name = sub () { $scalar }; 105 } else { 106 if (@_ == 1) { 107 my $scalar = $_[0]; 108 *$full_name = sub () { $scalar }; 109 } elsif (@_) { 110 my @list = @_; 111 *$full_name = sub () { @list }; 112 } else { 113 *$full_name = sub () { }; 114 } 115 } 116 } 117 } 118} 119 1201; 121 122__END__ 123 124=head1 NAME 125 126constant - Perl pragma to declare constants 127 128=head1 SYNOPSIS 129 130 use constant PI => 4 * atan2(1, 1); 131 use constant DEBUG => 0; 132 133 print "Pi equals ", PI, "...\n" if DEBUG; 134 135 use constant { 136 SEC => 0, 137 MIN => 1, 138 HOUR => 2, 139 MDAY => 3, 140 MON => 4, 141 YEAR => 5, 142 WDAY => 6, 143 YDAY => 7, 144 ISDST => 8, 145 }; 146 147 use constant WEEKDAYS => qw( 148 Sunday Monday Tuesday Wednesday Thursday Friday Saturday 149 ); 150 151 print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n"; 152 153=head1 DESCRIPTION 154 155This will declare a symbol to be a constant with the given value. 156 157When you declare a constant such as C<PI> using the method shown 158above, each machine your script runs upon can have as many digits 159of accuracy as it can use. Also, your program will be easier to 160read, more likely to be maintained (and maintained correctly), and 161far less likely to send a space probe to the wrong planet because 162nobody noticed the one equation in which you wrote C<3.14195>. 163 164When a constant is used in an expression, perl replaces it with its 165value at compile time, and may then optimize the expression further. 166In particular, any code in an C<if (CONSTANT)> block will be optimized 167away if the constant is false. 168 169=head1 NOTES 170 171As with all C<use> directives, defining a constant happens at 172compile time. Thus, it's probably not correct to put a constant 173declaration inside of a conditional statement (like C<if ($foo) 174{ use constant ... }>). 175 176Constants defined using this module cannot be interpolated into 177strings like variables. However, concatenation works just fine: 178 179 print "Pi equals PI...\n"; # WRONG: does not expand "PI" 180 print "Pi equals ".PI."...\n"; # right 181 182Even though a reference may be declared as a constant, the reference may 183point to data which may be changed, as this code shows. 184 185 use constant ARRAY => [ 1,2,3,4 ]; 186 print ARRAY->[1]; 187 ARRAY->[1] = " be changed"; 188 print ARRAY->[1]; 189 190Dereferencing constant references incorrectly (such as using an array 191subscript on a constant hash reference, or vice versa) will be trapped at 192compile time. 193 194Constants belong to the package they are defined in. To refer to a 195constant defined in another package, specify the full package name, as 196in C<Some::Package::CONSTANT>. Constants may be exported by modules, 197and may also be called as either class or instance methods, that is, 198as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where 199C<$obj> is an instance of C<Some::Package>. Subclasses may define 200their own constants to override those in their base class. 201 202The use of all caps for constant names is merely a convention, 203although it is recommended in order to make constants stand out 204and to help avoid collisions with other barewords, keywords, and 205subroutine names. Constant names must begin with a letter or 206underscore. Names beginning with a double underscore are reserved. Some 207poor choices for names will generate warnings, if warnings are enabled at 208compile time. 209 210=head2 List constants 211 212Constants may be lists of more (or less) than one value. A constant 213with no values evaluates to C<undef> in scalar context. Note that 214constants with more than one value do I<not> return their last value in 215scalar context as one might expect. They currently return the number 216of values, but B<this may change in the future>. Do not use constants 217with multiple values in scalar context. 218 219B<NOTE:> This implies that the expression defining the value of a 220constant is evaluated in list context. This may produce surprises: 221 222 use constant TIMESTAMP => localtime; # WRONG! 223 use constant TIMESTAMP => scalar localtime; # right 224 225The first line above defines C<TIMESTAMP> as a 9-element list, as 226returned by localtime() in list context. To set it to the string 227returned by localtime() in scalar context, an explicit C<scalar> 228keyword is required. 229 230List constants are lists, not arrays. To index or slice them, they 231must be placed in parentheses. 232 233 my @workdays = WEEKDAYS[1 .. 5]; # WRONG! 234 my @workdays = (WEEKDAYS)[1 .. 5]; # right 235 236=head2 Defining multiple constants at once 237 238Instead of writing multiple C<use constant> statements, you may define 239multiple constants in a single statement by giving, instead of the 240constant name, a reference to a hash where the keys are the names of 241the constants to be defined. Obviously, all constants defined using 242this method must have a single value. 243 244 use constant { 245 FOO => "A single value", 246 BAR => "This", "won't", "work!", # Error! 247 }; 248 249This is a fundamental limitation of the way hashes are constructed in 250Perl. The error messages produced when this happens will often be 251quite cryptic -- in the worst case there may be none at all, and 252you'll only later find that something is broken. 253 254When defining multiple constants, you cannot use the values of other 255constants defined in the same declaration. This is because the 256calling package doesn't know about any constant within that group 257until I<after> the C<use> statement is finished. 258 259 use constant { 260 BITMASK => 0xAFBAEBA8, 261 NEGMASK => ~BITMASK, # Error! 262 }; 263 264=head2 Magic constants 265 266Magical values and references can be made into constants at compile 267time, allowing for way cool stuff like this. (These error numbers 268aren't totally portable, alas.) 269 270 use constant E2BIG => ($! = 7); 271 print E2BIG, "\n"; # something like "Arg list too long" 272 print 0+E2BIG, "\n"; # "7" 273 274You can't produce a tied constant by giving a tied scalar as the 275value. References to tied variables, however, can be used as 276constants without any problems. 277 278=head1 TECHNICAL NOTES 279 280In the current implementation, scalar constants are actually 281inlinable subroutines. As of version 5.004 of Perl, the appropriate 282scalar constant is inserted directly in place of some subroutine 283calls, thereby saving the overhead of a subroutine call. See 284L<perlsub/"Constant Functions"> for details about how and when this 285happens. 286 287In the rare case in which you need to discover at run time whether a 288particular constant has been declared via this module, you may use 289this function to examine the hash C<%constant::declared>. If the given 290constant name does not include a package name, the current package is 291used. 292 293 sub declared ($) { 294 use constant 1.01; # don't omit this! 295 my $name = shift; 296 $name =~ s/^::/main::/; 297 my $pkg = caller; 298 my $full_name = $name =~ /::/ ? $name : "${pkg}::$name"; 299 $constant::declared{$full_name}; 300 } 301 302=head1 BUGS 303 304In the current version of Perl, list constants are not inlined 305and some symbols may be redefined without generating a warning. 306 307It is not possible to have a subroutine or a keyword with the same 308name as a constant in the same package. This is probably a Good Thing. 309 310A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT 311ENV INC SIG> is not allowed anywhere but in package C<main::>, for 312technical reasons. 313 314Unlike constants in some languages, these cannot be overridden 315on the command line or via environment variables. 316 317You can get into trouble if you use constants in a context which 318automatically quotes barewords (as is true for any subroutine call). 319For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will 320be interpreted as a string. Use C<$hash{CONSTANT()}> or 321C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from 322kicking in. Similarly, since the C<< => >> operator quotes a bareword 323immediately to its left, you have to say C<< CONSTANT() => 'value' >> 324(or simply use a comma in place of the big arrow) instead of 325C<< CONSTANT => 'value' >>. 326 327=head1 AUTHOR 328 329Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from 330many other folks. 331 332Multiple constant declarations at once added by Casey West, 333E<lt>F<casey@geeknest.com>E<gt>. 334 335Documentation mostly rewritten by Ilmari Karonen, 336E<lt>F<perl@itz.pp.sci.fi>E<gt>. 337 338=head1 COPYRIGHT 339 340Copyright (C) 1997, 1999 Tom Phoenix 341 342This module is free software; you can redistribute it or modify it 343under the same terms as Perl itself. 344 345=cut 346